Steam generator and coal pulverizing apparatus



Sept. 16, 1969 c. GARRETT, JR 3,467,036

STEAM GENERATOR AND COAL PULVERIZING APPARATUS Filed Dec. 5/ 1967 2Sheets-Sheet 2 F ICE- 2 L k L M IL A J A f/VVZ/V70A? Cfl/QA 05 JGAZQ/PE7ZJQ ZZ/f 57 United States Patent O 3,467,036 STEAM GENERATOR ANDCOAL PULVERIZING APPARATUS Carlos J. Garrett, Jr., Granby, Conn,assignor to Combustion Engineering, Inc, Windsor, Conn., a corporationof Delaware Filed Dec. 5, 1967, Ser. No. 688,046 Int. Cl. F23k 3/02;F22b 37/42; B02c 4/00 US. Cl. 110-106 12 Claims ABSTRACT OF THEDISCLOSURE An air swept coal pulverizer and classifier adapted for rapidresponse to the demands of a steam generator. The classifier receivesair within entrained coal particles from the pulverizer, separates thecoarse from the fine coal particles entrained in the air, allows thefine particles to pass to the steam generator and returns the coarseparticles to the pulverizer. Coal feeders to the pulverizer areresponsive to a control signal indicating the coal feed requirements ofthe steam generator. The classifier is temporarily adjusted to decreasethe quantity rejection of coarse particles in response to the rate ofchange of an increasing control signal. Air flow through the pulverizermay also be increased in response to the rate of change to thisincreasing control signal.

BACKGROUND OF THE INVENTION This invention relates to the application ofa coal pulverizer to the firing of a steam generator and in particularto an apparatus for improving the response time of coal supply to thesteam generator in response to a change in demand for coal feed.

Large steam generators are commonly fired with pulverized coal by whatis called the direct firing system. Crushed coal is fed into apulverizer as required by the steam generator with the coal then beingpulverized and immediately supplied directly to the steam generator. Thechange in firing rate to the steam generator is controlled by regulatingthe rate of coal feed to the pulverizer.

The pulverized coal particles are swept from the pulverizer by a fiow ofgas, such as air. This gas stream along with the entrained coalparticles are passed through a classifier which separates the fineparticles of coal from the coarse particles. The coarse particles arereturned to the pulverizer for further grinding while the fine particlesare conveyed with the gas stream to the steam generator furnace. Theseclassifier settings are generally manually adjustable so that they maybe set for the proper coal fineness for the best over-all operation. Ifthe coal passing to the furnace is too coarse, combustion is delayed andparticles of coal are incompletely burned resulting in an unburnedcarbon loss which lowers the steam generator efficiency. If on the otherhand, the coal is too finely ground, this results in excessive powerconsumption by the pulverizer and excessive wear. The classifiersettings are therefore adjusted for a desired coal fineness andreadjusted only when required due to wear of pulverizer and classifierparts or by a change in the type of coal being fired.

Steam generators when operating in conjunction with turbo generators arefrequently required to make rapid load changes while maintainingparameters such as temperature and pressure at a stable level. Delay inthe reaction time of inputs to the steam generator, such as fuel feed,increase the difiiculty of rapid response to load changes and limit therate at which a load change can safely be made.

When a change in fuel feed is required for a steam generator, the rateof feed to the pulverizer is changed.

There is no significant change in the output from the pulverizer untilabout a minute later. In order for the pulverizer to grind coal at theincreased rate, there must be a larger quantity of coal within thepulverizer itself and therefore the output rate is not increased untilthe coal storage within the pulverizer has increased to a new level. Itis known that the rate of grinding of a pulverizer is a function of thefineness to which the coal must be ground as well as the amount of coalwhich is inside the pulverizer within reasonable operating limits.

Some attempt to increase the response of coal delivery from thepulverizer has been made in the past by changing the air flow throughthe pulverizer simultaneously with the increase in coal feed. The rangeof air flow available is limited since too low an air flow will permitcoal settling in the horizontal runs of pipe while too high an airvelocity may cause the flame in the furnace to move so far from theburner as to permit a loss of the fires. While the increase in air flowthrough the pulverizer will operate to increase the amount of coalentrained in the air stream, there is also an increase in the airvelocity passing through the classifier and so the dynamic forcesincrease, resulting in a rejection of most of the coarse particles whichhave been entrained due to the increased air flow. The gain due to thisair flow therefore is minimal.

SUMMARY OF THE INVENTION An adjustable coal classifier is suppliedbetween a pulverizer and a steam generator furnace. This classifier isadjusted in response to the steam generator fuel feed demand signal inaddition to the conventional response wherein fuel feed to thepulverizer is adjusted. This provides an immediate increase in thedelivery of coal to the furnace since the classifier is adjusted topermit the passage of coarser coal particles. This takes coal from theamount which would otherwise be returned to the pulverizer. Thepulverizer will also sustain a higher grinding rate for the coarserrequirements established by the new classifier setting so that deliveryfrom the pulverizer can be sustained at a higher rate consistent withthe increased fuel feed to the pulverizer. The classifying means arethen gradually returned to the original position to provide for thedelivery of coal of the proper fineness to the furnace. During this timeperiod the coal storage within the pulverizer can be built up tomaintain the higher grinding rate at normal fineness.

Conversely, on the decrease in demand, the classifier is adjusted toreject more coal, permitting a similar and immediate response in theload decrease direction. While variation in fineness to the furnace willresult in a high unburned carbon loss for a load increase and higherpower consumption for a load decrease, these are only temporaryconditions and are insignificant compared to the increased ability torapidly control the steam generating system.

The air flow through the pulverizer may be simultaneously increased toincrease the amount of coal picked up from the pulverizer. The change inclassifier setting permits the increased coal picked up by this air flowto be passed on to the steam generator for immediate load response.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic diagram ofasteam generator and turbo generator combination showing the controlsystem for regulating the coal pulverizing and feeding apparatus;

FIGURE 2 is a sectional side elevation of the coal pulverizer andclassifier; and

FIGURE 3 is a plan view of the pulverizer and classifier indicating themechanism for simultaneously adjusting the classifier settings.

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3 DESCRIPTION OF THE PREFERRED EMBODIMENT Steam generator 2 has afurnace 3 fired by burners 4. The steam generator has tubular heatingsurface 5 through which water to be evaporated is forced by feed pump 7and controlled by feedwater valve 8. Steam line 9 conveys the steam tothe turbine 10 through the turbine throttle valve 12. The turbine isdirectly connected to an electric generator 13 with a watt meter 14measuring the electrical output of the system.

A flow of secondary air 15 is supplied through air control dampers 17 tosupport combustion in the furnace. A flow of primary air 18 is forced byfan 19 through the pulverizer 20, classifier 22 and the control damper23 through coal pipe 21 to the burners 4. This air entrains coal frompulverizer and conveys it to the furnace 3. A supply of coal 24 is fedby coal feeder 25 to pulverizer 20 at a rate which during steady stateoperation is identical with the coal feed requirements of the steamgenerator. This crushed coal is pulverized to a fine powder inpulverizer 20 and conveyed by the primary air flow 18 to the classifier22. The fine coal is passed through the classifier to the furnace whilethe coarser particles are rejected and returned through line 27 to thepulverizer for further grinding.

A signal indicating the actual generator output is passed throughcontrol line 28 to set point 29 where it is compared with a desiredgeneration signal 30. A fuel demand signal is passed through controlline 32 with the signal being at a level to indicate any change in fuelinput required to the steam generator. Other control signals (not shown)are used to regulate the feedwater flow to the steam generator as wellas to the turbine throttle valve position. Since this invention dealswith fuel feed to the steam generator, these other control signals arenot considered. The illustration in FIGURE 1 is that of a once-throughsteam generator which is the type in which rapid control response isparticularly essential. If this system were used on a drum type steamgenerator, the primary fuel feed signal would normally come from boilerpressure rather than generator output.

This control signal is then modified by control element 33 whichincorporates proportional, integrating, and differential action into thecontrol signal to an extent required for the particular characteristicsof the steam generator and turbine systems. This modified fuel demandsignal then passes through control line 34 with identical signals beingpassed through control lines 35 to air fiow controller 36 and controllines 37 and 38 to coal feeder controller 39. Since the air fuel ratiomust be maintained approximately constant, the secondary air flow isadjusted in proportion to the fuel feed. Controller 36 operates tophysically move dampers 17 regulating the flow of primary air 15 whilecontroller 39 operates to adjust the speed of coal feeder 25 regulatingthe rate of introduction of coal 24 to the pulverizer 20. When aircontrol damper 17 is adjusted, there is very little delay in the changeof air flow to the steam generator. The only delay is due toacceleration of the air, compression of the air where pressure mustchange, and for certain types of control the inertia of a fan where itsspeed must be changed. There is however substantial delay between thetime that the rate of feed of coal through feeder 25 is changed and thetime at which this increased coal feed is reflected on the delivery rateof coal through coal pipe 21 to the furnace. While there is sometransport time delay involved, the most significant delay is thatrequired to increase the coal storage in the pulverizer to a point whichattains equilibrium at the new higher grinding rate.

In order to eliminate or diminish this delay, the fuel feed demandsignal is simultaneously passed through control line 40 to controlelement 42 where it is modified with derivative action. This derivativeaction, which is commonly called rate action, produces a signal which isindicative of the rate of change of the control signal; and this rateaction signal is passed through control line 43 to classifier controller44. This control signal operates to adjust the classifier settingsimultaneously with any change in coal feeder speed. The change in theclassifier setting is a function of the rate of change of the coalfeeder speed, with the control signal through control line 43 decreasingslowly when there is no change in the rate of feeder speed, so thesetting of classifier 22 returns to its original setting slowly duringstable operation.

FIGURE 2 illustrates a typical pulverizer wherein coal is fed throughpipe 42 into a rotating bowl 53. The coal introduced into this bowl iscrushed between a stationary but rotating roller 54 to effectpulverization. A gas such as secondary air is supplied underneath thebowl through an opening 55 and passes upwardly through annular spaces 57sweeping fine coal from the outer edges of the rotating bowl 53 andentraining it in the air flow. This air along with the entrained coalparticles is passed upwardly to the classifier 22 and must flow radiallyinward through openings 58. Adjustable vanes 59 impart a rotating motionto the air flow which must pass around deflector ring 60 and upwardlythrough the air outlet 62. While the fine particles entrained in the airstream are able to remain entrained while the air fiow makes thesechanges in direction, the coarser particles are thrown outwardly anddownwardly being returned through an opening 63 to the rotating bowl 53.

Adjusting arms 64 are directly connected to the adjustable vanes 59 sothat the vanes may be adjusted to regulate the speed of rotation of theair flowing through the pulverizer. When the vanes are adjusted closerto the radial position, there is decreased spin, and a higher percentageof the coarse particles are passed through the classifier. Conversely asthe vanes approach a tangential position, this spin is increased with anincreased rejection of the coal through opening 63.

A ring linkage 65 engages all of the adjusting arms 64 so that they maybe moved simultaneously. Classifier controller 44 operates on this ringlinkage to adjust the position of classifier vanes 59. As previouslydescribed, an increased coal demand signal not only operates oncontroller 39 to increase the coal feeder speed, but also sends acontrol signal through control line 40 and a rate signal through controlline 43 to controller 44. This controller 44 then rotates controllinkage 65 to the left so as to move the control vanes 59 closer to theradial position. This immediately decreases the amount of coal rejectedby classifier 22, thereby increasing the amount of coal passing throughoutlet 62 and decreasing the amount of coal being returned throughopening 63. Instant response is therefore obtained in fuel flow to thefurnace without an increase in coal storage in bowl 53 which would berequired to sustain the higher grinding rate at normal fineness. Sincethe pulverizer during this time is required to grind less finely, theincreased feed rate can be sustained. Since it is undesirable tocontinuously operate the steam generator with unnecessarily coarse coal,the adjustable vanes 59 are allowed to slowly drift back to the originalposition with coal storage in the rotating bowl 53 being increasedduring this time period.

While the adjustment of the classifier adjustable vanes accomplish anincrease in pulverizer response, an even better response can be obtainedby simultaneously and temporarily adjusting the flow of primary airthrough the pulverizer. This is accomplished by regulating the aircontrol damper 23 which may be located either downstream of thepulverizer or upstream of the pulverizer where it need handle only cleanair. To accomplish this the rate signal passing through control line 43is also passed through control line 67 to primary air fiow controller 68which operates to increase the primary air flow through the pulverizertemporarily when there is a demand for coal flow increase to the steamgenerator. This increases the air fiow velocities within the pulverizerthereby entraining more pulverized coal in the air fiow. While theincreased air velocity through the classifier 22 would normally reject avery large percentage of the increased amount of coal entrained, incombination with the previously described adjustment of the classifieradjustable vanes 59, this increased supply of coal is enabled to remainentrained in the air. The increased coal supply is therefore immediatelyconveyed to the furnace for rapid response to a required load change.

The description of the preferred embodiment has considered an increasein coal demand to the furnace. Increased response to a load decreaserequirement is also obtained by the apparatus of my invention. Theadjustable vanes 59 are moved in the opposite direction so as toincrease the amount of rejection and return to the bowl 53 at the sametime that there is a decrease in the speed of the coal feeder 25. Thispermits the bowl 53 to gradually decrease the storage of coal thereinwhile the adjustable vanes 59 slowly return to their original position.

While I have illustrated and described a preferred embodiment of myinvention it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madetherein without departing from the spirit and scope of the invention. Itherefore do not wish to be limited to the precise details set forth butdesire to avail myself of such changes as fall Within the purview of myinvention.

What I claim is:

1. An apparatus for supplying pulverized coal to a steam generatorfurnace comprising: a coal pulverizing means; adjustable feed means forsupplying coal to said pulverizer at a first rate; means forestablishing a fiow of gas through said pulverizing means and to thefurnace, said gas flow entraining particles of coal within saidpulverizer and conveying them to the furnace; adjustable classifyingmeans for separating coarse particles from the coal particles entrainedin said gas flow, located intermediate said pulverizing means and saidfurnace; means for returning the coarse particles to said pulverizer;means for establishing a control signal indicative of a desired fuelfeed input to the steam generator furnace; and means for adjusting saidfeed means to a second rate in response to said control signal; meansfor simultaneously adjusting said classifying means in response to saidcontrol signal, in a direction to increase the coarseness of particlesreturning to said pulverizer in response to a control signal indicatingan increased desired fuel feed input.

2. An apparatus as in claim 1 wherein said means for adjusting saidclassifying means in response to said control signal also comprises:means for adjusting said classifying means in such a direction as todecrease the coarseness of particles returned to said pulverizer inresponse to a control signal indicating a decreased desired fuel feedinput.

3. An apparatus as in claim 1 having a modified control signal which isthe derivative action ofsaid control signal; and wherein said means foradjusting said classifying means operates in response to said modifiedcontrol signal.

4. An apparatus as in claim 3 wherein said means for adjusting saidclassifying means in response to said modified control signal comprises:means for adjusting said classifying means in a direction to decreasethe coarseness of particles returned to said pulverizing means inresponse to the modified control signal indicative of a decreaseddesired fuel feed input.

5. An apparatus as in claim 3 having also means for returning saidclassifying means to the original position during a predetermined timeperiod.

6. An apparatus as in claim 5 wherein said means for adjusting saidclassifying means in response to said modified control signal comprises:means for adjusting said classifying means in a direction to decreasethe coarseness of particles returned to said pulverizing means inresponse to the modified control signal indicative of a decreaseddesired fuel feed input.

7. An apparatus as in claim 1 having also: regulating means forregulating the gas flow through said pulverizer means; and means forsimultanously adjusting said gas flow regulating means in a direction toincrease the gas flow in response to a control signal indicating anincreased desired fuel feed input.

8. An apparatus as in claim 2 having also: regulating means forregulating the gas flow through said pulverizer means; means forsimultaneously adjusting said gas flow regulating means in a directionto increase the gas flow in response to a control signal indicating anincreased desired fuel feed input; and means for adjusting said gas flowregulating means in such a direction as to decrease the gas flow inresponse to a control signal indicating a desired decreased fuel feedinput.

9. An apparatus as in claim 3 having also: regulating means forregulating the gas flow through said pulverizer means; and means foradjusting said gas flow regulating means in response to said modifiedcontrol signal.

10. An apparatus as in claim 4 having also: regulating means forregulating the gas flow through said pulverizer means; means forregulating said gas flow in such a direction as to increase the gas flowin response to a modified control signal indicating an increased desiredfuel feed input; and a means for adjusting said gas flow regulatingmeans in such a direction as to decrease the gas flow in response to amodified control signal indicative of a decreased desired fuel feedinput.

11. An apparatus as in claim 9 having also: means for returning saidclassifying means to the original position during a predetermined timeperiod; and means for returning said gas flow regulating means to aposition corresponding with the original gas flow rate during apredetermined time period.

12. An apparatus as in claim 10 having also: means for returning saidclassifying means to the original position during a predetermined timeperiod; and means for returning said gas flow regulating means to aposition corresponding with the original gas fiow rate during apredetermined time period.

References Cited UNITED STATES PATENTS 2,841,125 7/1958 Falla. 3,205,8439/1965 Bogot -106 JAMES W. WESTHAVER, Primary Examiner US. Cl. X.R.122--449

